Two different subtypes of cannabinoid receptors (CB1 and CB2) have been isolated and both belong to G protein coupled receptor superfamily. Alternative spliced forms of CB1, CB1A and CB1B have also been described, but are expressed only at low levels in the tissues tested. (D. Shire, C. Carrillon, M. Kaghad, B. Calandra, M. Rinaldi-Carmona, G. Le Fur, D. Caput, P. Ferrara, J. Biol. Chem. 270 (8) (1995) 3726–31; E. Ryberg, H. K. Vu, N. Larsson, T. Groblewski, S. Hjorth, T. Elebring, S. Sjögren, P. J. Greasley, FEBS Lett. 579 (2005) 259–264). The CB1 receptor is mainly located in the brain and to a lesser extent in several peripheral organs, whereas the CB2 receptor is predominately distributed in the periphery primarily localized in spleen and cells of the immune system (S. Munro, K. L. Thomas, M. Abu-Shaar, Nature 365 (1993) 61-61). Therefore in order to avoid side effects a CB1-selective compound is desirable.
Δ9-tetrahydrocannabinol (Δ9-THC) is the principal psychoactive compound in the Indian hemp (Y. Gaoni, R. Mechoulam, J. Am. Chem. Soc., 86 (1964) 1646), cannabis sativa (marijuanan), which is used in medicine since ages (R. Mechoulam (Ed.) in “Cannabinoids as therapeutic Agents”, 1986, pp. 1–20, CRC Press). Δ9-THC is a non-selective CB1/2 receptor agonist and is available in the USA as dronabinol (marinol®) for the alleviation of cancer chemotherapy-induced emesis (CIE) and the reversal of body weight loss experienced by AIDS patients through appetite stimulation. In the UK Nabolinone (LY-109514, Cesamet®), a synthetic analogue of Δ9-THC, is used for CIE (R. G. Pertwee, Pharmaceut. Sci. 3 (11) (1997) 539–545, E. M. Williamson, F. J. Evans, Drugs 60 (6) (2000) 1303–1314).
Anandamide (arachidonylethanolamide) was identified as the endogenous ligand (agonist) for the CB1 receptor (R. G. Pertwee, Curr. Med. Chem., 6 (8) (1999) 635–664; W. A. Devane, L. Hanus, A. Breuer, R. G. Pertwee, L. A. Stevenson, G. Griffin, D. Gibson, A. Mandelbaum, A. Etinger, R. Mechoulam, Science 258 (1992) 1946–9). Anandamide and 2-arachidonoylglycerol (2-AG) modulate at the presynaptic nerve terminal negatively adenylate cyclase and voltage-sensitive Ca2+ channels and activates the inwardly rectifying K+ channel (V. Di Marzo, D. Melck, T. Bisogno, L. De Petrocellis, Trends in Neuroscience 21 (12) (1998) 521–8), thereby affecting neurotransmitter release and/or action, which decreases the release of neurotransmitter (A. C. Porter, C. C. Felder, Pharmacol. Ther., 90 (1) (2001) 45–60).
Anandamide as A9-THC also increases feeding through CB1 receptor-mediated mechanism. CB1 receptor selective antagonists block the increase in feeding associated with administration of anandamide (C. M. Williams, T. C. Kirkham, Psychopharmacology 143 (3) (1999) 315–317; C. C. Felder, E. M. Briley, J. Axelrod, J. T. Simpson, K. Mackie, W. A. Devane, Proc. Natl. Acad. Sci. U. S. A. 90 (16) (1993) 7656–60) and caused appetite suppression and weight loss (G. Colombo, R. Agabio, G. Diaz, C. Lobina, R. Reali, G. L. Gessa, Life Sci. 63 (8) (1998) L113–PL117).
Leptin is the primary signal through which the hypothalamus senses nutritional state and modulates food intake and energy balance. Following temporary food restriction, CB1 receptor knockout mice eat less than their wild-type littermates, and the CB1 antagonist SR141716A reduces food intake in wild-type but not knockout mice. Furthermore, defective leptin signaling is associated with elevated hypothalamic, but not cerebellar, levels of endocannabinoids in obese db/db and ob/ob mice and Zucker rats. Acute leptin treatment of normal rats and ob/ob mice reduces anandamide and 2-arachidonoyl glycerol in the hypothalamus. These findings indicate that endocannabinoids in the hypothalamus may tonically activate CB1 receptors to maintain food intake and form part of the neural circuitry regulated by leptin (V. Di Marzo, S. K. Goparaju, L. Wang, J. Liu, S. Bitkai, Z. Jarai, F. Fezza, G. I. Miura, R. D. Palmiter, T. Sugiura, G. Kunos, Nature 410 (6830) 822–825).
It has also been reported that the CB1 receptor plays a role in the regulation of bone mass and bone loss resulting from estrogen deficiency. Antagonists of CB1 and CB2 receptors prevented ovariectomy-induced bone loss in vivo and caused osteoclast inhibition in vitro by promoting osteoclast apoptosis and inhibiting production of several osteoclast survival factors (A. I. Idris, R. J. van't Hof, I. R. Greig, S. A. Ridge, D. Baker, R. A. Ross, S. H. Wilson, Nature Medicine 11 (7) (2005), 774–779). Cannabinoid receptor antagonists can therefore be useful for the treatment of osteoporosis and other bone diseases such as cancer associated bone disease and Paget's disease of bone.
At least two CB1 selective antagonist inverse agonists (SR-141716 and SLV-319) are currently undergoing clinical trials for the treatment of obesity and/or smoking cessation. In a double blind placebo-controlled study, at the doses of 10 and 20 mg daily, SR 141716 significantly reduced body weight when compared to placebo (F. Barth, M. Rinaldi-Carmona, M. Arnone, H. Heshmati, G. Le Fur, “Cannabinoid antagonists: From research tools to potential new drugs.” Abstracts of Papers, 222nd ACS National Meeting, Chicago, Ill., United States, Aug. 26–30, 2001). SR-141716 reduced body weight, waist circumference and improved metabolic parameters (plasma HDL, triglycerides and insulin sensitivity) in several phase III studies (RIO-lipids, RIO-Europe and RIO-North America). Additionally SR-141716 has shown efficacy in a phase III trial for smoking cessation (STRATUS-US). There still remains a need for potent low molecular weight CB1 modulators that have pharmacokinetic and pharmacodynamic properties suitable for use as human pharmaceuticals.
Other compounds which have been proposed as CB1 receptor antagonists/inverse agonists are aminoalkylindoles (AAI; M. Pacheco, S. R. Childers, R. Arnold, F. Casiano, S. J. Ward, J. Pharmacol. Exp. Ther. 257 (1) (1991) 170–183), like 6-bromo-(WIN54661; F. M. Casiano, R. Arnold, D. Haycock, J. Kuster, S. J. Ward, NIDA Res. Monogr. 105 (1991) 295–6) or 6-iodopravadoline (AM630, K. Hosohata, R. M. Quack, R. M; Hosohata, T. H. Burkey, A. Makriyannis, P. Consroe, W. R. Roeske, H. I. Yamamura, Life Sci. 61 (1997) 115–118; R. Pertwee, G. Griffin, S. Fernando, X. Li, A. Hill, A. Makriyannis, Life Sci. 56 (23–24) (1995) 1949–55). Arylbenzo[b]thiophene and benzo[b]furan (LY320135, C. C. Felder, K. E. Joyce, E. M. Briley, M. Glass, K. P. Mackie, K. J. Fahey, G. J. Cullinan, D. C. Hunden, D. W. Johnson, M. O. Chaney, G. A. Koppel, M. Brownstein, J. Pharmacol. Exp. Ther. 284 (1) (1998) 291–7) disclosed in WO9602248, U.S. Pat. No. 5,596,106, 3-alkyl-(5,5-diphenyl)imidazolidinediones (M. Kanyonyo, S. J. Govaerts, E. Hermans, J. H. Poupaert, D. M. Lambert, Bioorg. Med. Chem. Lett. 9 (15) (1999) 2233–2236.) as well as 3-alkyl-5-arylimidazolidinediones (F. Ooms, J. Wouters, O. Oscaro. T. Happaerts, G. Bouchard, P. -A. Carrupt, B. Testa, D. M. Lambert, J. Med. Chem. 45 (9) (2002) 1748–1756) are known to antagonize the CB1 receptor respectively act as an inverse agonist on the hCB1 receptor. WO0015609 (FR2783246-A1), WO0164634 (FR2805817-A1), WO0228346, WO0164632 (FR2805818-A1), WO0164633 (FR2805810-A1) disclosed substituted 1-bis(aryl)methyl-azetidines derivatives as antagonists of CB1. In WO0170700, WO02076949, and WO0276949A1 4,5-dihydro-1H-pyrazole derivatives are described as CB1 antagonists. In several patents and publications bridged and non-bridged 3-pyrazolecarboxamide derivatives are disclosed as CB1 antagonists/inverse agonists (WO0132663, WO0046209, WO9719063, EP658546, EP656354, U.S. Pat. No. 5,624,941, EP576357, U.S. Pat. No. 3,940,418, WO03020217, WO0335005, J. M. Mussinu et al., Bioorg. Med. Chem. 2003, 11, 251; S. Ruiu et al., J. Pharm. Expt. Ther., 2003, 306, 363). Pyrrole CB1 cannabinoid receptor agonists have been described in G. Tarzia et al., Bioorg. Med. Chem. 2003, 11, 3965. Phenethyl amides have been claimed as CB1 cannabinoid receptor antagonists/inverse agonists in WO03077847, WO03082190, WO03086288 and WO03087037. Various aza heterocycles (imidazoles, triazoles and thiazoles) are described in WO0337332, WO03040107, WO0306378 1, WO03082833 and WO03078413. Diphenylpyrazine carboxamides are described in WO03051850, diphenylpyridine carboxamides in WO03084930 and diphenylbenzene carboxamides in WO03084943.